Catalysts mixed-metal carbonyl clusters

Fig. 24. Structures of mixed metal carbonyl clusters that are potential catalysts eflective in CO hydrogenation to produce C and Cj alcohols.

In addition, to facilitating the preparation of highly dispersed iron catalysts, the use of iron carbonyls and carbon supports facilitates the preparation of promoted catalysts to increase the selectivity to olefines. Thus, the use of mixed-metal carbonyl clusters as metal precursors allows the preparation of a variety of stoichiometric metal compositions, something difficult to reach by co-impregnation techniques. Furthermore, the mixed-metal carbonyl cluster should be activated by heating just to that minimum temperature which would decompose the cluster to yield reduced metal and CO, the temperature being <475 K. Hence, it is possible to obtain reduced metals under much less severe conditions than those used for conventional metal salt precursors. 

Several mixed-metal molecular cluster carbonyls that have recently been used as precursors for heterogeneous catalysts (e.g. Nasher et al.2 and Shephard et al.,3 In the former... 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

Ruthenium-Rhodium Bimetallic Catalysis. In seeking to inqprove the ethylene glycol syntheses of Table 1, one possibility that has not been extensively studied until recently (46-49), is the use of mixed metal centers with bimetallic, polymetallic or bridged-metal carbonyl clusters either as catalyst precursors, or generated in situ. 

An alternative method of preparation of bimetallic catalysts [11-14] is based on the direct interaction of metal carbonyl clusters with surfaces of supported metals under mild conditions. In this work, a comparison is made of various methods of preparation of mixed metallic particles by interaction of organometallic compounds with metal surfaces. 

In this body of catalysts, the metal cluster is said to be formed around the carbonyl precursor. According to SEM and TEM imaging, it appears that the carbonyl clusters are on the order of 1 pm in diameter when supported on carbon.192 Analysis with FTIR has shown that the carbonyl is present.189 190 198-200 203 Non-noble metals have also been studied along side the noble-metals in this group of catalysts. Table 4 lists the non-noble metal carbonyl catalysts studied.189-192 198-200 The non-noble metal carbonyl catalysts studied produced mixed results for the ORR activity. 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

The water-gas-shift reaction catalysed homogeneously in the presence of polynuclear metal carbonyls is of current interest. In some ruthenium systems, the principal species present in basic solutions under reaction conditions of one atmosphere pressure and at 100°C are [HRu3(CO)n] and [H3Ru4(CO)i2] . This has reasonably been taken as evidence to implicate ruthenium clusters as the probable catalysts, although it should be noted that mononuclear systems effectively promote the water-gas-shift reaction. An important finding is that mixed ruthenium-iron carbonyl clusters, e.g.. 

More recent developments in cluster chemistry have included the anchoring of clusters to silica or alumina surfaces via sol-gel processing, the products finding applications as catalyst precursors. Very recently, mixed-metal clusters have been incorporated onto the inner walls of mesoporous silica with a pore diameter of about 30 A, and have been subsequently converted into discrete nanoparticles by thermolysis, which have been shown to act as hydrogenation catalysts. This is a particularly important new use for mixed-metal cluster systems, since there are many well characterized mixed-metal cluster carbonyls vide infra) that are readily available to act as precursors for such reactions. 

Then they prepared mixed-metal cluster H2FeRu3(CO)i3 and evaluated for homogeneous WGS reaction [16]. The mixed-metal cluster H2FeRu3(CO)i3 is more active than either ruthenium carbonyl or iron carbonyl individually. This synergetic behaviour of the mixed-metal system is also observed for catalyst solutions prepared initially from Ru3(CO)i2 plus FefCOls or Fe3(CO)i2 or H4Ru4(CO)i2 plus Fe(CO)5 in alkaline solution. The catalytic activity of these... 

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